There were 20 people riding on a train before it stopped and some number of people got off. Now there are 2 people riding on the train. How many people got off the train at the stop?
Solution: The number of people who got off is the difference between the number who were riding before and the number who are riding now. The difference is $20 - 2$ people. $20 - 2 = 18$ people got off.